Fix SC.Module loading on IE11

IE11 removed support for onreadystatechange.
SC.Module was assuming that IE would always use onreadystatechange, so the module loaded callback would never get called on IE11.

I changed this to a feature detect to pick between using addEventListener on the load event, and onreadystatechange if supported.

Details on the recommended feature detect here: http://msdn.microsoft.com/en-us/library/ie/hh180173(v=vs.85).aspx